Anodic oxidation in which the components are connected as anodes and, together with an opposite electrode, are exposed to d.c. voltage, has been known for a long time (Praktische Galvanotechnik, Eugen G. Leuze-Verlag, Saulgaul/Wurtt., 4.sup.th Edition, 1984). Already during the stage of its formation, the anodically precipitating oxygen combines with the aluminium surface and the structure and thickness of the thus formed layer depends primarily on the duration of the process and the current density. By this process, the colorless oxide film which always forms on an aluminium surface in an atmosphere of air, is thickened to a dimension of up to 100 .mu.m so that a high corrosion resistance against oxidizing media and a high resistance against wear mechanisms is established.
A drawback of this, however, is that, in the case of aluminium alloys of the aluminium silicon type, the differing microstructural constituents with their differing electric conductivity lead to the formation of an oxide layer of non-uniform thickness which can be disadvantageous for certain technical uses.